A combustion problem in internal combustion engines can be, for example, the occurrence of so-called knocking combustion. With knocking combustion, the ignition of the fuel/air mixture first takes place normally via the sparking of a spark plug, with a flame front forming in the usual way. The, as a rule, high temperature and pressure increase in the cylinder can cause a self-ignition to occur at another point of the cylinder, which is extremely heated in an unwanted manner, with the development of a second flame front. The two flame fronts then collide with each other and cause a characteristic knocking noise.
In the case of an—unwanted—extreme heating of a point of the cylinder, it is also possible for so-called premature ignition or ignition by incandescence to occur, which in an unwanted manner initiates an ignition before the actual ignition point. Premature ignition, which is another combustion problem, can have a more destructive effect than knocking combustion, since premature ignition causes the temperature at the piston head to rise extremely so that the melting point of the piston material is reached after a few seconds only and hence the piston can be destroyed. In the case of premature ignition, the ignition point can no longer be controlled by the engine control. Parts subject to extreme heating in this way include discharge valves, spark plugs, sealing parts and deposits on these parts and the surfaces surrounding the combustion chamber.
To prevent the occurrence of combustion problems such as, for example, knocking combustion, the publication DE 10 2006 015 662 A1, for example, suggests using so-called knock sensors to detect noises in the internal combustion engine, with the noises for each combustion cycle serving to form cylinder-individual signals. Hereby, a monitored cylinder is switched off, i.e., for example, the fuel supply to this cylinder is interrupted when knocking due to premature ignition is present, i.e. when the current signal values which are specific for this cylinder exceed prespecified setpoint values.
Furthermore, a combustion problem can also manifest itself in a change to the so-called segment times of the crankshaft. The segment times describe the times required by the crankshaft during the working cycles of the individual cylinders to pass through prespecified angular degrees.
The measurement of the segment times for the detection of irregular running is described for example in the publication DE 19544720 C1. Here, in dependence on the measured segment times and after subjecting the measured segment times to correction factors reflecting mechanical inaccuracies of a rotational speed sensor for the measurement of the segment times, an irregular running value is determined and compared with prespecified setpoint values. On the basis of the comparison, corresponding changes are then made to operating parameters of the respective cylinders in order to suppress the irregular running.
It has been found to be a drawback of the method described in the prior art that it is not possible to achieve adequate recognition of the different types of combustion problems.